Cyclic oligosiloxanes are used as the raw material for high molecular weight polysiloxanes such as silicone fluid and gum, and when having Si—H bonds, as the raw material for organohydrogenpolysiloxanes, the raw material for adhesive aids having organic functional groups bonded thereto, and the like.
One process known in the art for the preparation of cyclic oligosiloxanes is by hydrolysis of a silane having two hydrolyzable groups such as dimethyldichlorosilane, as described in JP-A 54-74900 and U.S. Pat. No. 4,447,630 (JP-A 60-90220). When it is desired to selectively produce a cyclic oligosiloxane during hydrolysis, the co-presence of alcohol during hydrolysis is a common practice, leaving the problems of low pot yields and increased BOD loads due to the inclusion of alcohol in the water discharge. When the desired cyclic oligosiloxane is recovered from the silane hydrolyzate, fractional distillation is necessary, resulting in an extended manufacture process.
It is known from JP-A 2000-159783, JP-A 2000-159784, and JP-A 2000-169488 that cyclic oligosiloxanes are produced through disproportionation reaction of high molecular weight siloxanes in the presence of catalysts.
The methods described in these patents use metal alkoxides as the catalyst for helping produce cyclic oligosiloxanes under milder conditions. During the process, however, molecules having a silicon-alkoxide bond originating from the metal alkoxide catalyst form in addition to the cyclic oligosiloxane. When the oligosiloxane produced is used as the raw material, these molecules become impurities adversely affecting the properties of the end product. A high-performance fractional distillation unit is necessary for separating the molecules having a silicon-alkoxide bond from the cyclic oligosiloxane.
JP-A 7-316167 discloses the use of anhydrous aluminum chloride catalyst for producing cyclic tetramer siloxane. The anhydrous aluminum chloride suffers from extreme difficulty of handling and a likelihood of generating hydrochloric acid in the system.